Click on My Account, which is located under your name in the dashboard. You’ll see your Billing History at the bottom of this page:

On the billing history page, click on the blue Cancel link to cancel your subscription:

You’ll be asked to confirm that you wish to cancel your subscription:

When you cancel your subscription, your account will remain a Professional or Corporate account until your annual or monthly renewal date. When the annual or monthly renewal date passes, your account will be downgraded to a free account. You won’t lose any data, though you will lose the ability to export your data and will have limited access to reports. You’ll lose the ability to create custom styles, and will be allowed 200 responses per month for surveys and quizzes.